The present invention relates to an interconnection system and method for heterogeneous networks, and in particular, to an interconnection system and method for heterogeneous networks in which a terminal of a local area network (LAN) and a terminal of an integrated services digital network (ISDN) can be interconnected with each other only through a translation of a communication path identifier of an open system interconnection (OSI) layer by means of an LAN/ISDN inter-working unit.
In heterogeneous network systems, in order to allow communications to be accomplished between two terminals, namely, between an LAN terminal connected to an LAN and an ISDN terminal linked with an ISDN via an LAN/ISDN inter-working unit, it is necessary to conduct a translation between an LAN protocol and an ISDN protocol in the inter-working unit. Since a method of specifying a communication path for the OSI layer varies between the LAN and the ISDN, the communication path specification method is required to be translated, which is one of the primary protocol translation operations.
In accordance with the Institute of Electric & Electronics Engineers (IEEE), the LAN protocol is stipulated to conform to the IEEE standards 802.2 to 802.5. In the standards, a communication path is specified by a pair of an address of an LAN terminal as a data destination and an address of an LAN terminal as a data transmitting source. Furthermore, the LAN terminal address is represented by a combination of an address of a media access control (MAC; destination and source addresses are respectively abbreviated as DA and SA) and an access point of a logical link control (LLC; destination and source access points are respectively denoted as DSAP and SSAP). In consequence, the LAN terminal provides the OSI layer with a communication path designated as "DA, SA, DSAP, SSAP". Namely, a communication frame assigned with these addresses is transferred through an LAN communication path.
On the other hand, a draft of a standardized protocol for an efficient packet communication in the ISDN are currently being discussed as an additional packet mode bearer service (APMBS) in the International Consultive Committee for Telephone and Telegraph (CCITT). The APMBS is characterized in that the data link control procedure called the link access procedure on the D-channel (LAPD) adopted for the D channel of the ISDN in the present stage is expanded to be applicable to other channels so as to multiplex a plurality of data link connections on ISDN channels. In communication conforming to a protocol stipulated as the APMBS, a communication path is denoted by use of a set of an ISDN channel number and a data link connection identifier (DLCI). Since the ISDN channel number is identified in a physical layer, the ISDN line is supplied with a frame including the LAPD header containing the DLCI, which will be described later in conjunction with FIG. 4B.
Heretofore, the interconnecting system associated with the LAN and the ISDN has been described, for example, in an article, Dieter Japel, et al., "LAN/ISDN Interconnect Via Frame Relay" published in Session 54.2 of the 1988 GLOBECOM.
FIG. 1 shows the configuration of a network system in which an LAN 20 accommodating a plurality of LAN terminals 200-1 to 200j is interconnected via an LAN/ISDN inter-working unit 100 to an ISDN 30 connecting a plurality of ISDN terminals 300-1 to 300i. In the following description, the LAN 20 may be a single LAN or may be a multi-LAN including a plurality of LANs interconnected to each other by means of a bridge. This is also the case of the ISDN 30. Furthermore, each LAN terminal 200 is provided with an LAN protocol standardized in conformity with the IEEE, whereas each ISDN terminal 300 supports a frame relay protocol selected according to the CCITT APMBS.
In the network system of FIG. 1, for example, in order to enable the LAN terminal 200-j to communicate with the ISDN terminal 300-i, the LAN/ISDN inter-working unit 100 is required to posses the following functions.
Namely, in a frame communication with the LAN terminal, the LAN/ISDN inter-working unit 100 functions, as shown in FIG. 2A, such that the communication is achieved via a bridge 110 with a pseudo LAN 22 accommodating pseudo LAN terminals 310-1 to 310-i associated with the ISDN terminals 300-1 to 300-i. On the other hand, when achieving a frame communication with the ISDN terminal, the LAN/ISDN inter-working unit 100 operates, as shown in FIG. 2B, so that the communication is achieved via an ISDN inter-connector 120 with a pseudo ISDN 32 accommodating pseudo ISDN terminals 210-1 to 210-j related to the LAN terminals 200-1 to 210-j. For these two kinds of operations, the ISDN inter-working unit 100 is required to be provided with three function as shown in FIG. 2C, namely, the functions of the bridge 110, an LAN/ISDN protocol converter 130 (for an address translation and a route or routing control), and the ISDN inter-connector 120.
The address translation and routing control unit, namely, the LAN/ISDN protocol converter 130 converts the communication path identifier (DA, SA, DSAP, SSAP) in a frame received from the LAN 20 into an ISDN communication path indicator (ISDN channel number, DLCI) to send the received frame to the ISDN inter-connector 120 processing in association with the ISDN channel number. In this situation, the DA is designated by an MAC address of a pseudo LAN terminal beforehand registered to the ISDN inter-connector 120. Conversely, on receiving a frame from the ISDN 30, the converter 130 translates an ISDN communication path indicator denoted by an DLCI and a reception channel number assigned to the frame into an LAN communication path identifier to transfer the reception frame to the bridge 110.
For the conversion between the LAN and ISDN communication paths, the LAN/ISDN inter-working unit 100 is required to retrieve a table 190 of FIG. 3.
The table 190 comprises, for a definition of an LAN communication path, a field 191 indicating an MAC address DAi of a destination or called LAN terminal i, a field 192 denoting an MAC address SAj of a calling LAN terminal j, a field 193 designating an LCC service access point (SAPi) of the destination LAN terminal DAi, and a field 194 indicating LLC.multidot.SAP of the calling LAN terminal DAj. Furthermore, the table 190 includes, for a definition of an ISDN communication path, a field 195 denoting an ISDN channel number P.sub.l in the LAN/ISDN inter-working unit 100 and a field 196 designating a data link connection identifier DLCI independently assigned to each ISDN channel. A plurality of data link connections are multiplexed on each ISDN channel such that each data link connection is identified depending on the DLCI.
In this configuration, however, since the table 190 has a size determined by the number of combinations each including the DA, SA, DSAP, and SSAP, the capacity thereof becomes greater in association with an increase in the numbers respectively of the LAN and ISDN terminals communicating with each other. Moreover, at present, the MAC address of the LAN is generally 48 bit long, and such addresses are beforehand written in an read-only memory (ROM) at a delivery of the LAN terminal. Consequently, the user may be required to adopt these addresses in some cases. It is hence difficult to restrict only the MAC addresses of the LAN terminal to be communicated with the ISDN terminal to take successive values in a particular range.
In an interconnection between the LAN and the ISDN, when converting a communication path on the LAN side designated by a combination (DA, SA, DSAP, SSAP) of the MAC and LLC addresses assigned to an LAN frame into a communication path on the ISDN side indicated by a set (ISDN channel, DLCI) of a physical address and a data link identifier of the ISDN associated with the LAN, the retrieval of the table containing relations therebetween takes a considerably long period of time.
In other words, when retrieving the conversion table to obtain an entry (DAi, SAj, DSAPi, SSAPj) for a communication path identifier (DA, SA, DSAP, SSAP) assigned to a frame from the LAN, a long period of time is required for the LAN/ISDN inter-working unit to check a lot of entries stored in the conversion table. As a method of reducing the retrieval period of time, the DAs and SAs may possibly be loaded in the table at relative entry addresses having correspondences with the DAs and SAs. However, as described above, the DAs and SAs each are 48 bit long and the values thereof, particularly, the values of SAs cannot be easily limited to a particular range. In consequence, if the table is not of quite a large capacity, the method above cannot be considered to be practically feasible.